Hair-like filaments called pili enable some bacteria to remove uranium from contaminated groundwater. The discovery, published today in Proceedings of theNational Academy of Sciences, could aid in the development of radioactivity clean-up technologies.

Some bacteria, including a species called Geobacter sulfurreducens, are known to get their energy from reducing — or adding electrons to — metals in the environment. When uranium dissolved in groundwater is reduced in this way, the metal becomes much less soluble, reducing the spread of contamination.

Researchers have been trying to find out how the process works. They suspected that the pili might be the answer, but because G. sulfurreducens produces pili only in certain environments, the process has proved tricky to study.

Key to the discovery was getting Geobacter to make pili under lab conditions, for example by lowering the temperature. "Standard culture conditions are like a five-star hotel for Geobacter," says Gemma Reguera of Michigan State University in East Lansing, who led the research. "We had to make life a little rougher for them."

Reguera and her team were then able to show that the pili greatly increase the amount of uranium that G. sulfurreducens is able to remove. Without pili, the bacterium reduces uranium within the cell envelope, but this poisons the cell in the process. When pili are present, however, most of the precipitation occurs around the pili, which extend away from the cell. This provides a greater surface area for electron transfer, say the researchers, as well as keeping the radioactive uranium at a safe distance.

An electrifying tale
"This work ties a lot of things together," says Derek Lovley, a microbiologist at the University of Massachusetts Amherst and Reguera's former postdoctoral supervisor.

Earlier this year, Lovley published a paper in Nature Nanotechnology showing that the pili on G. sulfurreducens are a type of 'nanowire', because they conduct electricity. The pili help to power the bacterium by transferring electrons produced during the cell's metabolism to external acceptors such as iron. The fact that pili can also reduce a metal such as uranium "provides further evidence for long-range electron transfer along the pili", he says.

The research should help to improve bioremediation — the use of biological organisms to remove pollutants from soil and water — such as clean-up of the many sites contaminated by uranium processing during the cold war. "Current methods to stimulate the growth of these bacteria in the environment are pretty crude and empirical," says Lovley. "This new mechanism will allow us to better predict how uranium can be depleted."

Reguera is most excited about the possibility of "getting away from the bugs" and making non-living devices based on nanowires. "This would allow us to work in sites where bacteria cannot live," she says, such as the Fukushima nuclear plant in Japan, which was devastated by a tsunami earlier this year.

Uranium is not the main radioisotope released at Fukushima, but Reguera sees potential for widening the reach of Geobacter pili. In theory, she says, they could help to precipitate out the radioactive isotopes of other elements, such as technetium, plutonium and cobalt. Reguera also envisages fine-tuning the properties of the pili: "Because these nanofilaments are made from protein, we can easily add different functional groups," she says.

Microbiologist Yuri Gorby of the University of Southern California in Los Angeles is optimistic about an emerging field that he refers to as "electromicrobiology". He points out that other microbes, such as photosynthetic cyanobacteria and thermophilic methanogens, also produce conductive nanowires. "I believe that we have only just begun to scratch the surface," he says.

This article is reproduced with permission from Nature magazine. It was first published on September 5, 2011.

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